To stimulate purchase desires of consumers, in addition to pursuing outstanding function performance, manufacturers of industrial products provide the products in diversified novel appearance designs that are not only appealing to consumers but also enhance added values of the products. Taking a casing of an electronic device as an example, casings in earlier days are commonly made of plastic materials through an injection molding process. Although having advantages of being light in weight and high in formability, plastics suffer from an issue of unsatisfactory heat dissipation and are also likely to leave consumers an impression of an inferior quality. Therefore, certain manufacturers of making casings replace plastic materials by metal such as a magnesium alloy, an aluminum alloy or a titanium alloy. However, a coupling structure cannot be directly formed on the casing made of metal for coupling with an external element (e.g., a circuit board or a bracket). Alternatively, the coupling structure can only be disposed onto the casing by means of adhesion or welding as disclosed in Taiwan Patent Nos. M331441 and 420969 as well as Taiwan Publication Nos. 201134366 and 201134653. In the above disclosures, although an issue of not being able to directly form a coupling structure is solved, adhesion or welding of the coupling structure requires extremely precise positioning to ensure that an external element can be steadily coupled with the casing, which makes the manufacturing process of the casing complex and lengthens manufacturing time thereof.